`ament_copyright --add-copyright-year` broken

The `ament_copyright --add-copyright-year` tool is currently broken.

- https://github.com/ament/ament_lint/blob/rolling/ament_copyright/ament_copyright/main.py#L277
this attribute should be named `copyright_identifiers` (with an `s` at the end)
- https://github.com/ament/ament_lint/blob/rolling/ament_copyright/ament_copyright/main.py#L290
the function `search_copyright_information` returns 2 values, not 3

Besides these obvious bugs, I'm also unsure that the `add_copyright_year` function should parse the copyright comments.
the parsed data should already be available in the descriptor objects.

fixing these bugs should be easy.
however, it looks like the tool supports multiple copyrights. How do we deal with them?

Let's say that I have a file with 2 copyrights

```
# Copyright 2021 TheNewCompany

# Copyright 1999 TheOldCompany
```

IMO we should have the tool update only the latest copyright (or maybe the first one found in the document)